You can have several email ticket systems in the same project.
When you open the Email sub-tab of the Project Settings tab, you see a dropdown select list (with all ticket systems on the project) and a link to create a new email ticket system.
You can switch between ticket systems in the combo, and the data in the form below will update to reflect the selected ticket system.
Note that there must be at least one ticket system with the flag "default".
This function is useful when you have one support project, but want to use different products that have different email addresses.
Note: When you have multiple email ticket systems, one of them must have the "default" flag. The default ticket system cannot be deleted, as there must be at least one ticket system on a project.